Specifically you need to copy conf/nxt-default.properties to conf/nxt.properties and change the line nxt.apiServerHost=127.0.0.1 to nxt.apiServerHost=0.0.0.0
Then it is just a matter of configuring your iptables to open this port (lookup instructions for your distro).
Of course exposing your port to your local network will make your wallet far more vulnerable so do so at your own risk. It would be safer to only open this port when you need it.